1. Field of the Invention
The present invention relates to a system, a method, and a program for detecting a location by receiving radio signals from multiple radio signal generation sources. More specifically, the present invention relates to a system, a method and a program for estimating location data, of a given place, associated with radio strengths, on the basis of known location data associated with radio strengths recorded in advance.
2. Description of Related Art
There has heretofore been a demand to track, in a room, movement of an object provided with a radio signal receiver. For example, a technique called trace analysis has been carried out in a supermarket by tracking movement of a special information terminal attached to a shopping cart to investigate relationships between products on shelves and products customers buy. Thereby, placement of products can be optimized in the store.
To carry out a trace analysis, radio signal generation sources are provided at different places in the store. While a customer moves around in the store while pushing the shopping cart, the information terminal receives radio signals from the respective radio signal generation sources at different time points. With the information on such time difference, the location of the shopping cart can be determined by means of a method called triangulation. However, simple application of the triangulation method often leads to extreme deterioration in the accuracy of the location measurement. Such deterioration is likely to occur especially in the case of a location measurement in an indoor environment containing a large number of reflectors and obstacles. This is because such a large number of reflectors and obstacles cause reflected waves to interfere with each other in some cases, or, block all the direct paths between a radio signal receiver and radio signal generation sources, thereby causing the radio signal receiver to receive only reflected waves.
In view of the above problem, Japanese Patent Application Laid-open Publication No. 2005-274363 discloses a technique for obtaining the most-probable location of a mobile object in the following manner. Specifically, the deviations of radio location detection means are measured in advance at predetermined locations by means of a method employing triangulation. Thereafter, by interpolating this measured deviation information, the deviations of the radio location detection means at general locations are estimated. Then, in an actual measurement, a radio detection location is corrected by using the result of the estimation to obtain the most-probable location of the mobile object.
However, the calculation method used in triangulation is relatively simple, and hence, a location cannot be measured highly accurately with the method using triangulation even when various kinds of correction means are employed.
In the field of machine learning, a technique called label propagation method is known. A fundamental document disclosing this technique is “Semi-supervised learning using Gaussian fields and harmonic functions,” Proceedings of the Twentieth International Conference on Machine Learning (ICML-2003), by Xiaojin Zhu, Zoubin Ghahramani, John Lafferty, Washington D.C., 2003 (hereinafter, referred to as Non-patent document 1).
Non-patent document 1 relates to a method for estimating a label for an unlabeled vertex in a weighted graph having labeled and unlabeled vertices, and proposes application examples of digit classification and text classification techniques. However, no satisfactory results were obtained by directly applying the disclosed method without modification, to estimate a location based on the strengths of radio signals received from multiple radio signal generation sources.